#! perl -slw
use strict;

sub checksum {
	my( $string ) = @_;

	my $chk_sum = 0;
	$chk_sum += $_ for unpack 'C*', substr $string, 0, -5;

	my $twos_comp = hex( substr $string, -5, 4 );
	return ( ( $chk_sum + $twos_comp ) & 0xFFFF ) ? 1 : 0;
}

print "$_ : ", checksum $_ for "\x019999FF1B\x03", "\x019998FF1B\x03";

__END__
P:\test>283701
?9999FF1B? : 0
?9998FF1B? : 1


##</code><code>##

9999 = 57 + 57 + 57 + 57 = 228.
~ 228 = -229 decimal = FF1B hex (16-bit)
Add the header and trailer characters = <SOH>9999FF1B<ETX>

##</code><code>##

1 + 57 + 57 + 57 + 57 = 229

##</code><code>##

FF1B = 65307

##</code><code>##

(65307 + 229) = 65536
65536 & 65535 = 0x10000 & 0xFFFF = 0

##</code><code>##

sub build_string {
	my( $string ) = @_;
	my $chksum = 0;
	$chksum += $_ for map{ $_ & 0x7F } unpack 'C*', $string;
	$chksum = (~$chksum & 0xFFFF);

	return chr(1) . $string . sprintf( '%4x', $chksum ) . chr(3);
}